This invention relates to liquid sensing arrangements, and more particularly to such arrangements using electromagnetic sensors.
In many situations, it is much more important to accurately know the amount of liquid in a container when the container is near empty than when it is near full. The problem is particularly acute where an undesirable event accompanies the exhaustion of the liquid and/or where contentional measuring techniques, such as a narrow bottom portion of a container in which the remaining liquid pools, do not work.
One example of where all the foregoing exist is in a satellite attitude control system employing gas jets or thrusters. Gas for the jets or fuel for the thrusters is usually stored in liquid form in containers or tanks on the satellite. When the gas or fuel is used up, most satellites become inoperative, or at least suffer significantly degraded capability. Moreover, without gravity to pool the liquid in the tank it tends to distribute itself in response to other forces. Fuel is generally used herein to denote the fluid contents of the container, whether or not the liquid would strictly be considered fuel.
The length of time that a particular load of fuel will provide stationkeeping for a satellite cannot be predicted exactly, because it depends upon environmental conditions, such as solar wind pressure and magnetic fluctuations. Furthermore, the exact amount of fuel used during each maneuver also depends upon the exact characteristics of controllers, which may vary slightly with time and environment. Thus, the exact amount of fuel remaining in a spacecraft becomes less well known as time progresses, unless a suitable measurement scheme is used.
At the end of life of a geostationary satellite, for example, the remaining fuel is used to remove it from its assigned orbital position so that a replacement satellite may be inserted into that orbital position. So long as the old satellite is given enough velocity to vacate the regions of interest, it is irrelevant how much additional velocity it is given. If the amount of fuel remaining in the tanks at the time it is ejected from its orbital position exceeds that necessary, the excess fuel could have been used for additional useful time in orbit. Because the amount of fuel required for stationkeeping is very small, months of potential stationkeeping time could be wasted if the ejection from orbital position occurs too early. On the other hand, if the decision is made too late, there may be insufficient fuel to cause the satellite to be removed from its orbital position, and the orbital position it occupies may not be usable for a replacement satellite.
Communication satellite weight is tightly controlled in order to provide the largest possible load of fuel for stationkeeping, thereby obtaining the longest possible useful life. For a satellite with 24 transponders which are polarization-multiplexed to provide 48 effective communication channels, the value of additional usable fuel may be several million dollars per additional month of useful life.
Thus, it is very advantageous to be able to determine the amount of remaining fuel. The measurement, however, is rendered complex because under weightless conditions, the fuel spreads through the tank. In order to prevent the fuel from forming into balls spread throughout the tank, the tanks include one or more thin circumferential bands near the inner surface of the tank. These bands capture the central bubble which tends to form, holding the remaining fuel against the outer walls and the bands.
Since the remaining fuel near the end of life has a relatively small volume compared with the volume of the tank, measurement methods which compare volume lack sensitivity near the end of spacecraft life, when the tank approaches empty, which is when the accuracy is most needed. Similarly, methods which depend on the mass of fuel lack sensitivity because of the increasing effect of the mass of the tank and surrounding objects. The extremely irregular shape of the fuel as it adheres by surface tension to the tank and the circumferential bands further complicates the problem. Yet further, the size and weight of a fuel sensor should not be so great that the additional fuel which it displaces from the spacecraft exceeds the additional amount of fuel rendered usable by knowledge of the actual amount of remaining fuel.